In recent years, X-ray CT scanners have made significant improvement and have been widely used for computerized tomography. The use of the scanner is not only in the medical field, but also in other fields including industrial applications. The X-ray CT scanners have also made rapid advancements. For example, in response to the demands from the medical community for high resolution and wide scanning range, multi-slice X-ray CT scanners have been developed for wide use. The multi-slice X-ray CT scanner generally includes an X-ray source for radiating a fan beam X-ray in a slice direction and two-dimensional X-ray detector elements also placed in the slice direction. The slice direction is generally the longitudinal axis of a scanning bed where a patient lies. The X-ray detector includes 4, 8, 16 or 32 rows of the detector elements as commercially available from Toshiba Medical Systems in Aquilion Models. As will be later described in detail, the scan operation is performed in a multi-scan mode or a helical-scan mode in the CT scanner. Due to the above advancement, the three-dimensional image data is obtained in a shorter period of time at higher accuracy over a more extended area of the patient in comparison to the single-slice X-ray CT scanner.
The above obtained image data is reconstructed in three dimension for not only display but also for other various purposes. For example, one medical purpose is to measure the clot size or the occlusion rate due to stenosis in the blood vessels. To perform the above measurements, after a patient is injected with a contrast agent, the three-dimensional image data is obtained via the X-ray CT scanner for imaging the distribution of the contrast agent flowing in the blood vessels. Based upon the distributed CT values of the contrast agent that is reflected in the three-dimensional image data, the clot size and the occlusion rate are actually measured. In case of the occlusion rate measurement, the rate is determined based upon the comparison of the internal thickness or distance of the vessel at the normal area and at the occluded area as represented in the three dimensional image data or volume data. For the blood vessel thickness measurements, a predetermined threshold value is established for the CT values. In the above examples, although the above three-dimensional image data is obtained by the X-ray CT scanner, the three-dimensional image data is also obtained by other types of scanners.
Other types of scanners include ultrasound imaging scanners and magnetic resonance imaging scanners. To assuredly perform the blood vessel measurement for the displayed image, Japanese Patent Application 11-342132 discloses a blood vessel size measuring technique based upon the pixel value profile in the desired area that is perpendicular to the cross sectional area of the blood vessel. On the other hand, Japanese Patent Application 2000-350726 discloses a technique for accurately measuring the length of objects such as blood vessels and intestines having curves in the directions that are not parallel to the projection surface based upon the maximal intensity projection (MIP) image.
For the measurements or display of certain minute structures, the CT scanner systems generally experience a certain amount of blur, smear or inaccuracy in the three-dimensional image data. The blur in the image or the smeared-out image is caused by the limit in spatial resolution. The spatial resolution also depends upon the basic performance characteristics of the scanner as well as the scan conditions. For example, the basic performance characteristics of the scanner device include the pitch between two adjacent detector elements. The scan conditions include conditions under which a particular scan is performed or the three dimensional image is reconstructed. One example of the conditions includes a slice thickness of the scan. As the slice thickness is made larger along the body axis of the patient or the Z direction, the spatial resolution along the Z direction decreases. Consequently, the blur is caused in the Z direction or the body axis for a lower quality image. Ultimately, the inaccurate measurements of certain structures such as the thickness of blood vessel walls lead to unreliable information including the occlusion rate. Some aspects of a point spread function (PSF) are disclosed in “Imaging” PCT Application, WO 00/22573, Chui et al.
The super-resolution or corrective process is performed in predetermined directions with respect to the scanning direction. Assuming the scanning direction is the Z direction or the body axis, the blur in the CT image occurs in the X and Y directions that are perpendicular to the Z direction. However, the amount of blur or PSF differs in each direction in the three-dimensional image data. As the result of the differing spatial resolutions, the image quality is not stable.
For the above described problem, it is desired to substantially minimize the effect of the blur on the scanned three-dimensional image data for accurately measuring a certain structure of interest. In substantially eliminating the blur, it is also desired to apply a technique that is applicable to the three-dimensional image data that is scanned by various types of scanners and under varying conditions. Ultimately, it is desired to improve high-quality three-dimensional image data.